Hall effect devices are typically used to sense the presence, and sometimes the magnitude, of a magnetic field. A simplified schematic of a basic Hall effect device 100 is shown in FIG. 1 to illustrate the basic concept thereof.
The Hall effect device 100 generally has a Hall element, or plate, 102 through which a bias current (I_bias) is passed from a first bias terminal, or node, 104 to a second bias terminal 106. When the bias current I_bias is applied in the presence of a magnetic field perpendicular to the plane of the Hall element 102, the Lorenz force acting on the majority carriers in the Hall element 102 generates a “Hall voltage” (VH+ to VH−) in the form of an offset voltage across Hall voltage terminals 108 and 110. The Hall voltage is typically relatively small, so an amplifier 112 is commonly used to enhance the output (VH+ to VH−) of the Hall element 102. The output of the amplifier 112 may form the output of the overall Hall effect device 100 as an analog signal indicative of the strength of the magnetic field perpendicular to the Hall element 102. Alternatively, the output of the amplifier 112 may pass to an output driver 114 (such as a Schmitt Trigger) to drive an output transistor 116, which produces a binary (i.e. on/off, yes/no, true/false) output signal indicative of whether the Hall effect device 100 is within a magnetic field sufficient to trigger the output signal.
Additionally, a supply voltage VS is provided to the Hall effect device 100 to power the components thereof, typically through a bandgap regulator 118. Including a ground, the Hall effect device 100 is, thus, typically a three-pin component.
A problem with Hall effect devices (e.g. 100) is that the output voltage (VH+ to VH−) of the Hall element 102 typically varies with the temperature of the Hall element 102. Graphs 120, 122 and 124 in FIGS. 2, 3 and 4, respectively, illustrate this phenomenon, commonly called “temperature drift.” (The values shown in the graphs 120, 122 and 124 are typical for some situations, but are shown for illustrative purposes only.)
The voltage (V_bias1) at the first bias terminal 104 increases with temperature, while the voltage (V_bias2) at the second bias terminal 106 is generally fixed, relative to ground, as shown in FIG. 2. The voltage across the bias terminals 104 and 106, thus, increases with increasing temperature, as shown in FIG. 3. This change in voltage across the bias terminals 104 and 106 is generally due to a characteristic change of resistance in the material of the Hall element 102 over the illustrated temperature range, while the bias current I_bias is held relatively constant. The increase in the voltage V_bias1 at one of the bias terminals 104 results in an overall increase in the net velocity of the majority carriers in the material of the Hall element 102 due to the temperature change. Consequently, the Hall voltage (VH+ to VH−) also increases with temperature, as shown in FIG. 4.
With the values shown in FIGS. 2-4, the change in temperature results in about a five percent change in the Hall voltage (VH+ to VH−) over the given temperature range. Such a variation in the Hall voltage (VH+ to VH−) is unacceptably large for many applications.
Manufacturers of Hall effect devices have developed a variety of techniques for compensating for temperature drift. Such techniques have employed a variety of digital algorithms for adjusting the Hall voltage based on a measured temperature, various methods for modulating the bias current (so it is not a constant current) and diverse analog circuits for fine-tuning the output Hall voltage, among other techniques. Because of the need for additional components (not shown in FIG. 1) to perform the temperature compensation, these techniques have various tradeoffs with respect to advantages and disadvantages regarding size, complexity and cost of the Hall effect devices. Due to the very common usage of these devices in a wide variety of applications, it is highly desirable to develop smaller, simpler and cheaper Hall effect devices.
It is with respect to these and other background considerations that the present invention has evolved.